The present invention relates to a sediment remediation process, together with a material for use in a sediment remediation process.
The progressive eutrophication of estuarine and freshwater systems throughout Australia and overseas is often reflected in an increase in both phytoplankton bloom frequency and biomass, often with a shift to more nuisance species such as cyanobacteria. A significant proportion of the research into, and management of, estuarine and freshwater systems has focussed (with varying success) on reducing inputs of nutrients (in particular phosphorus) from the catchment.
However, the present invention has recognised that increased internal loadings derived from sedimentary nutrient stores accumulated over years to decades, constitute a major barrier to the effective management and restoration of estuarine and freshwater systems. Indeed, it is now apparent that if effective sediment-nutrient management strategies can be identified and adopted to modify internal nutrient (phosphorus) loadings, this will equip natural resource managers with a powerful tool for both short- and long-term estuarine and freshwater system management.
An aim of the present invention is to provide a sediment remediation process and a sediment remediation material which will assist in modifying, and in particular reducing, internal recycling sediment nutrient stores in estuarine and freshwater systems.
Broadly, the invention provides a method for remediating matter by removing oxyanion or phosphorus containing pollutants therefrom, the method comprising the step of contacting the matter with a substrate doped, cation-exchanged, or modified with, or otherwise having adsorbed complexing element(s) selected from Group IIIB and Group IVB elements.
The mechanism for reactions which occur in the substrate on addition of the complexing elements, and the chemical speciation of these elements are presently not well understood. By the term xe2x80x9cmodifiedxe2x80x9d which term will be used hereafter, it is intended to cover whatever the reaction mechanism may be, which includes modification of the substrate by doping and/or cation exchange, and/or adsorption of the complexing element by the substrate. In addition, by the term xe2x80x9ccomplexing element(s)xe2x80x9d, ionic moieties containing such complexing elements are included, in addition to complexing element(s) alone.
Typical matter may comprise sediments in waterways and catchments, effluent from sewage treatment plants (commercial and/or domestic), industry, aquaculture (commercial and/or domestic and/or agricultural), sediments in water supply impoundments (lakes, reserviors), sediments in constructed wetlands and stormwater detention basins or similar engineered or natural impoundments.
Typical pollutants envisaged include phosphorus containing compounds, anions generally which are capable of forming complexes, and in particular oxyanions such as in particular phosphates, but also arsenate, vanadate, chromate and selenate, tungstate, niobate, tantalate, and tellurate, amongst others, and peroxyanions inter-alia such as persulphate. It is also expected that the method may have application in removing pollutants such as organic chemical contaminants such as pesticides or herbicides or trace elements, although this is not the primary objective of the invention.
Generally, phosphorus will be removed as dissolved phosphates or orthophosphate. Phosphates exist as different species, depending upon pH and other solution physico-chemical parameters. Phosphorus is often present in polluted aqueous environments in insoluble forms, and is transformed to soluble phosphate species by various processes that can occur within the environment. Examples of insoluble phosphorus include organically-bound phosphate which may become aqueously soluble due to biogeochemical processes, or phosphorus held in inorganic forms such as in mineral form as in mineral apatite or fertilizer, or that bound to crystalline and/or amorphous Fe-Mn-oxyhydroxide species all of which may be released due to various biogeochemical processes.
The method may include in addition, adding a water soluble salt of the complexing element selected from Group IIIB and Group IVB elements, along with the modified substrate. This would be expected to give rise to an immediate reduction in pollutant levels due to formation of complexes with the soluble salt, leaving the remediation material for more long term reduction in pollutants.
Preferably the salt is a chloride salt or a nitrate salt or a mixture of chloride and nitrate salts of the complexing element.
The present invention also provides a remediation material for use in reducing oxyanion or phosphorus pollutant loadings in matter, the remediation material comprising a substrate doped, cation-exchanged or modified with, or having adsorbed a complexing element(s) selected from the Group IIIB and Group IVB elements.
The substrate may be any suitable substrate having a moderate to high cation exchange capacity (CEC)xe2x80x94a substrate having a CEC of greater than about 30 milliequivalents per 100 grams (meq/100 g) having a xe2x80x98moderatexe2x80x99 CEC, while a xe2x80x98highxe2x80x99 CEC substrate may have a CEC of greater than about 100 meq/100 g and commonly about 150meq/100 g or greater.
It is preferred that the substrate is a mineral substrate due to these being in many instances relatively inert and/or harmonious in the environment.
In the most preferred form of the invention, the mineral substrate is an expandable clay such as saponite, bentonite or vermiculite. These materials are regarded as expandable clays due to their ability to absorb waters of hydration into their internal structure which may change the basal (d-) spacing.
Alternatively, the mineral substrate may be a fibrous, chain-like related clay mineral such as attapulgite, sepiolite, or palygorsite.
However, it will be appreciated that materials of a similar nature to clays, clay-like minerals, or expandable clays, may also be satisfactory. These may include materials both natural or synthetic. For instance, zeolites have also been investigated for use as the mineral substrate of the invention, both naturally occurring zeolites and artificially synthesised zeolites. Zeolites are also commonly a moderate to high cation exchange capacity material and although they are aluminosilicate minerals like clays, they have a different three dimensional framework structure with internal cavities.
The mineral substrate of the invention preferably has a high CEC in order to allow it to be modified to increase its pollutant or nutrient binding capacity, and in particular its phosphorous binding/absorbing/complexing capacity. Specifically, this involves the exchange of cations present in the mineral substrate with the nutrient complexing element or elements referred to above. This modification may be described as doping or cation exchange/ion exchange.
The mineral substrate may be pre-treated with a concentrated acid (e.g. HCl, H2SO4) to remove a large proportion of the interlayer and/or structural cations, before being treated with the complexing element. The pretreatment of clays with acid represents another pathway to prepare modified clays for phosphate adsorption. A potential advantage of this technique is that there may be a degree of modification to the underlying clay structure which enhances the uptake of the complexing element or other structural changes to the clay. These structural changes may make the clay more amenable to other modification steps which may improve the phosphate uptake capacity.
The complexing element is preferably an element capable of forming a complex with oxyanions. Most preferably the complexing element is capable of forming a complex with phosphorus containing compounds, as phosphorus is often the most common nutrient present in contaminated aquatic systems which may mean there is a high potential for algal bloom growth. Typically, the phosphorus will be present as phosphate anions in such aquatic systems.
The remediation material may be applied as a dry powder, as pellets, or as a wet slurry to the surface of a waterbody, or directly to the surface of bottom sediments, or injected into the bottom sediments. It is advantageous to form a capping layer of remediation material to the surface of bottom sediments, water conditions such as flow rates and turbulence permitting. The capping layer may be of any thickness, but a range between 0.5 mm and 5 mm should prove suitable, with an optimum range between 2 mm and 3 mm begin suitable for most conditions, without giving rise to undesirable side effects to the existing ecosystems. The layer thickness required will depend on factors such as rate, duration, and variability of phosphorus release, the rate and/or capacity of adsorption/binding/complexation of the remediation material, the desired phosphorus reduction and the influence of any other environmental and/or physico-chemical conditions.
The remediation material may be sandwiched between geotextiles such as water permeable membranes or woven plastic cloth such as woven PVC (poly vinyl chloride) cloth. In the case of pelletised remediation material, a weave net having apertures smaller than the pellets may be used to sandwich the pellets, in order to accommodate a high solute flow while minimising loss of the remediation material.
The remediation material is believed to be particularly suitable for reducing internal phosphorus loadings in bottom sediments in estuarine or freshwater systems.
The element is preferably selected from the Group IIIB and IVB elements from the Periodic Table (CAS version). The Group IIIB elements comprise scandium, yttrium, lanthanum, and actinium, and for the purposes of this specification are deemed to include lanthanides. The Group IVB elements comprise titanium, zirconium, and hafnium. The lanthanides comprise lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thullum, ytterbium, and lutetium. A mixture of such elements may be used.
In particular, the element is most preferably selected from the group comprising lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu) and yttrium (Y) or the group comprising zirconium (Zr) and hafnium (Hf), with lanthanum being the element of choice. As discussed above, it will also be appreciated that some of the remaining elements referred to above will not be preferred due to toxicity problems. The most preferred elements are selected from Group IIIB, Group IVB, and lanthanides, and have an atomic number between 21 and 72 inclusive.
With particular reference to the use of lanthanum, it has been demonstrated that lanthanum forms an extremely stable, redox-insensitive complex with phosphorous under most common environmental conditions, making the phosphorous unavailable to phytoplankton in aquatic systems and thus, potentially reducing the magnitude and/or frequency of algal blooms. With the lanthanum bound in the substrate, the lanthanum phosphate complex is effectively immobilised. In addition zirconium also forms a useful cation-exchanged modified substrate. It is believed that a mixture of cations comprising lanthanum and/or zirconium, optionally with other rare earth elements may be used in the modified substrates.
The sediment remediation material of the present invention may be used as a reactive capping or layer on top of bottom sediments in estuarine or freshwater systems. Due to the high nutrient binding capacity of the sediment remediation material (and in the preferred form its phosphorus binding capacity), the layer of material binds and substantially reduces the availability of the internal store of nutrients in the sediment, thus reducing the waterway nutrient loading.
Thus, the present invention additionally provides a sediment remediation process, the process comprising placing a reactive capping or layer of a sediment remediation material on top of bottom sediments in estuarine or freshwater systems, the sediment remediation material comprising a mineral substrate doped, cation-exchanged, or modified with, or having adsorbed a nutrient complexing element(s) selected from the Group IIIB and Group IVB elements.
The sediment remediation material may also be altered by the addition of organic and/or inorganic ligands to the clay and/or to the interlayer ions thereof, to alter its chemical properties for a particular application. This can form complexes with the exchanged cation in the substrate, resulting in the modified behavior in the sediment remediation material.
The placement of the reactive capping or layer may be achieved using known equipment and apparatus. However, the form of the sediment remediation material may be physically and/or chemically altered to suit a particular application. For example, where the mineral substrate is an expandable clay, the clay may be heated for varying times and to varying temperatures (using a combination of temperature and time to achieve the desired effect) such that the clay is selectively and suitably dehydrated.
This dehydration step is believed to be beneficial because at various stages of hydration there may still be significant exchange (the magnitude of which may be controlled) of lanthanum (for instance) into solution. The release of lanthanum may also be modified either singly or in combination by changes in the physico-chemical conditions of the solution such as pH and/or ionic strength. This lanthanum, once released into solution, may then be available to bind phosphorous in the water mass, leaving remaining lanthanum in the clay to bind phosphorus from sediment or other sources. This form of the invention assists in binding phosphorous in both the water mass and the sediment, as the material settles down into the water to form the reactive capping.
This dehydration step may additionally be beneficial because at sufficiently high temperatures (ca. greater than 900-1000 deg C.) the clay structures may decompose to form a variety of other mineral phases. These degradation products may also be useful but are yet to be evaluated.
Alternatively the sediment remediation material may be pelletised and the pellets injected, placed upon the bottom by mechanical or other means (e.g. settling through the water column), or otherwise mixed with the bottom sediments. In one such application, it is expected that the element may be substantially immobilised within the expandable clay by driving off any water of hydration.
Alternatively, the physical form of the sediment remediation material may be modified, such as by pelletisation, to not only modify the movement of the material under a range of hydrodynamic scenarios but also to influence the release rate and/or availability of lanthanum (for instance) due to changes in the surface area of the pellets.
In can thus be seen that in the preferred form of the invention, the lanthanum doped, cation-exchanged, or modified clay or other mineral substrate has the potential to be used in a variety of natural aquatic environments (for examplexe2x80x94lake, river, estuarine), or artificial aquatic environments (for example farm dam, aquaculture and treated/untreated industrial and/or sewage effluent), to reduce the concentration of dissolved phosphorus and in turn algal growth and/or biomass and/or bacterial growth and/or biomass and/or biological oxygen demand (BOD). The sediment remediation material may also be used to adsorb/bind/complex a range of other anions/oxyanions in aquatic systems.
Extensive laboratory trials have been undertaken to evaluate a wide range of lanthanum doped, cation-exchanged, or modified materials under a wide range of physico-chemical conditions. Preliminary results suggest that a lanthanum modified saponite or a lanthanum modified bentonite (both commercially sourced expandable clays) are the most promising materials. These materials have been demonstrated in trials to reduce dissolved phosphorous concentrations by in excess of 90% under a range of salinities in small scale batch tests. Application of these materials to sediment cores obtained from the Swan River in Western Australian demonstrated a similar phosphorous reduction capacity relative to untreated sediment cores over a period of seven days.